The study proposes to study serum leptin levels in patients with Prader- Willi, Bardet-Biedl, Alstrom and Cohen syndromes to determine the relationship with body mass index (BMI), and then to compare with the levels of non-syndromic obese children, and with non-obese children. Insights into the mechanism of obesity may help to design interventions aimed at its prevention and treatment, and consequently have a significant impact on health. In addition, studying the mechanism of obesity may contribute to the understanding of the multi-organ involvement in these patients. Leptin is a recently discovered peptide hormone that is produced by adipocytes. It is postulated that leptin acts as a feedback signal to the brain in the regulation of body weight, by providing information about fat stores. It has been shown that adults and children with obesity, as measured by increased BMI, have an approximately five-fold elevation in plasma leptin levels when compared to normal BMI controls, suggesting a receptor or post receptor signaling defect in the brain. Plasma leptin levels will be measured in the GCRC core laboratory and correlated with BMI.